Wireless communication systems are well known in the art. Generally, such systems comprise communication stations, which transmit and receive wireless communication signals between each other. Depending upon the type of system, communication stations typically are one of two types of wireless units: one type is the base station (BS), and the other is the wireless transmit/receive unit (WTRU), which may be mobile.
The term base station (BS) as used herein includes, but is not limited to, a base station, access point, Node B, site controller, or other interfacing device in a wireless environment that provides WTRUs with wireless access to a network with which the access point is associated.
The term wireless transmit/receive unit (WTRU) as used herein includes, but is not limited to, a user equipment, mobile station, fixed or mobile subscriber unit, pager, or any other type of device capable of operating in a wireless environment. Such WTRUs include personal communication devices, such as phones, video phones, and Internet ready phones that have network connections. In addition, WTRUs include portable personal computing devices, such as PDAs and notebook computers with wireless modems that have similar network capabilities. WTRUs that are portable or can otherwise change location are referred to as mobile units.
Typically, a network of base stations is provided wherein each base station is capable of conducting concurrent wireless communications with appropriately configured WTRUs, as well as multiple appropriately configured base stations. Some WTRUs may alternatively be configured to conduct wireless communications directly between each other, i.e., without being relayed through a network via a base station. This is commonly called peer-to-peer wireless communications. Where a WTRU is configured to communicate directly with other WTRUs it may itself also be configured as and function as a base station. WTRUs can be configured for use in multiple networks, with both network and peer-to-peer communications capabilities.
One type of wireless system, called a wireless local area network (WLAN), can be configured to conduct wireless communications with WTRUs equipped with WLAN modems that are also able to conduct peer-to-peer communications with similarly equipped WTRUs. Currently, WLAN modems are being integrated into many traditional communicating and computing devices by manufacturers. For example, cellular phones, personal digital assistants, and laptop computers are being built with one or more WLAN modems.
Popular WLAN environments with one or more WLAN base stations, typically called access points (APs), are built according to the IEEE 802 family of standards. Access to these networks usually requires user authentication procedures. Protocols for such systems are presently being standardized in the WLAN technology area such as the framework of protocols provided in the IEEE 802 family of standards.
FIG. 1 illustrates a conventional wireless communication environment in which WTRUs conduct wireless communications via a network station, in this case an AP of a WLAN. The AP is connected with other network infrastructure of the WLAN such as an Access Controller (AC). The AP is shown as conducting communications with five WTRUs. The communications are coordinated and synchronized through the AP. Such a configuration is also called a basic service set (BSS) within WLAN contexts.
In the wireless cellular context, one current standard in widespread use is known as Global System for Mobile Telecommunications (GSM). This is considered as a so-called Second Generation mobile radio system standard (2G) and was followed by its revision (2.5G). General Packet Radio Service (GPRS) and Enhanced Data for GSM Evolution (EDGE) are examples of 2.5G technologies that offer relatively high speed data service on top of (2G) GSM networks. Each one of these standards sought to improve upon the prior standard with additional features and enhancements. In January 1998, the European Telecommunications Standard Institute—Special Mobile Group (ETSI SMG) agreed on a radio access scheme for Third Generation Radio Systems called Universal Mobile Telecommunications Systems (UMTS). To further implement the UMTS standard, the Third Generation Partnership Project (3GPP) was formed in December 1998. 3GPP continues to work on a common third generational mobile radio standard. In addition to the 3GPP standards, 3GPP2 standards are being developed that use Mobile IP in a Core Network for mobility.
Much of the development of wireless communication systems has been motivated by the desire to reduce communication errors, improve range and throughput, and minimize costs. Most recent advances have been made possible by exploiting diversity in the time, frequency and code dimensions of communication signals. U.S. Pat. No. 5,614,914, which issued on Mar. 25, 1997 and is assigned to the assignee of the present invention, is an example of utilizing diversity to improve wireless communications.
Since the mid 1990s, the development of Multiple-Input Multiple-Output (MIMO) systems has led to increases in throughput without increasing transmission power or bandwidth, by exploiting the spatial diversity of the wireless communication channel. Much research effort on MIMO systems has been directed towards finding methods to improve throughput or signal to noise ratio (SNR) with given wireless channel conditions. Prior research has focused on using either space time coding (STC) or so-called water-filling applications over time, frequency, and space. The present invention utilizes a different approach, improving throughput and SNR by conditioning the channel. This approach can be used independently or in combination with other methods.
The improvement of spatial diversity presents a significant challenge. Increasing spatial diversity in MIMO systems may be exploited to achieve higher throughput for a given transmit power and bandwidth than Single-Input Single-Output (SISO), Single-Input Multiple-Output (SIMO) or Multiple-Input Single-Output (MISO) systems. A common technique for increasing spatial diversity is by physically relocating, moving, and/or adding antennas to the system. However this is not always possible. When it is not possible to add more antennas or adjust their positions, a typical MIMO system's ability to exploit available spatial diversity in the channel is limited to the channel condition defined by the physical configuration of the transmit and receive antennas. The inventors have recognized that is possible to improve communications in spite of such limitation and have devised the present invention for conditioning the channel.